Complementary metal oxide semiconductor (CMOS) image sensors (CIS) are manufactured using CMOS technology. In backside illuminated (BSI) CISs radiation sensed by the BSI CIS does not propagate through metal and dielectric layers.
FIG. 1 includes various cross sections that illustrate the manufacturing process of a prior art BSI CIS.
The manufacturing process may include:                a. Manufacturing a sensor wafer 20 that may be an epitaxial (epi) sensor wafer. The sensor wafer 20 may include a bulk (Bulk Si) 21, an active Silicon (Si) layer 22 which is an epitaxial grown Si, dielectric and metal layers (“back end”) 23 and first oxide adhesion layer 24.        b. Manufacturing a carrier wafer 30 which includes Si bulk 31 and a second oxide adhesion layer 32.        c. Bonding the sensor wafer 20 to the carrier wafer 30 to form wafer 40 by attaching the first and second oxide adhesion layers 24 and 32 to each other.        d. Removing the Si bulk 21 to expose active Si layer 22.        e. On top of the active Si layer 22 adding layers such as micro lenses 41, color filters (CFA) 42 and anti-reflective coating (ARC) layer 43 by a process that may include passivation, deposition of anti-reflecting coating, fabricating color filters, fabricating micro-lenses, and pad opening.        
ARC layer 43 prevents the reflection of light that propagates towards the active Si layer.
In several applications, and especially for three dimensional imaging, there is a need for high resolution sensors (and thus small pixels sensor) which have good quantum efficiency (QE) in the Near Infrared (NIR) and good modulation transfer function (MTF)—providing no crosstalk between pixels.
These requirements usually contradict each other since in NIR, the Si absorption coefficient is small, and a quite thick (much more than 10 microns) active Si layer is needed.
Pixels with dimensions that are significantly smaller than the active Si layer thickness are prone to severe crosstalk due to lateral diffusion of the generated carriers.
Furthermore, a typical request for such sensors is to be sensitive in a very narrow and specific wavelength, namely the light emitting diode (LED) and/or laser wavelength which this sensor is planned to detect. Other wavelengths are considered “background” illumination which has no contribution and in contrary add noise.
An external narrow band optical filter is usually used to avoid it.
Two known solution to the MTF/QE strong tradeoff are using high resistivity epitaxial material (epi) and using black Silicon.
Using high resistivity material includes forming the sensor on a thick epi, but with very low doping level (high resistivity epi). This makes depletion regions much larger and the fields formed by the photodiode penetrate deep into the epi layer. The carriers are swept towards the surface by the field rather than by diffusion. This reduces significantly the cross-talk.
Black Silicon is formed by processing the surface of the Si in order to change the direction of the incident photons. The black Silicon becomes a good NIR absorber even for shallow Si layer. Nevertheless, the issue of cross-talk remains and must be treated separately.
There is a growing need to provide a BSI CIS that exhibit high resolution, good quantum efficiency in NIR and good MTF—providing no crosstalk between pixels.